Association Between Substantia Nigra Hyperechogenicity and Central Macular Thickness in Parkinson’s Disease

Background/Objectives: This study investigates the correlation between substantia nigra (SN) hyperechogenicity, detected via transcranial sonography (TCS), and retinal thinning in patients with Parkinson’s disease (PD). Methods: In this cross-sectional study, 86 PD patients (172 eyes) underwent clinical assessment (UPDRS, Hoehn–Yahr), TCS for SN area measurement, and optical coherence tomography (OCT) for retinal nerve fiber layer (RNFL), ganglion cell-inner plexiform layer (GCIPL), and macular thickness analysis. Results: PD patients exhibited significant thinning of the GCIPL, average RNFL, and central macular thickness compared to healthy controls (p < 0.001 in all instances). Pathological SN hyperechogenicity (>0.19 cm2) was detected in 85.9% of patients with a preserved temporal bone window (71 out of 86). A significant negative correlation was found specifically between the SN echogenic area and central macular thickness (r = −0.248, p = 0.003). Patients with pathological SN findings had a significantly thinner central macula (p = 0.006) compared to those with a normal SN, while no significant correlations were observed between the SN area and RNFL or GCIPL. Conclusions: These findings demonstrate a specific correlation between SN hyperechogenicity and central macular thinning. While RNFL and GCIPL effectively differentiate PD from controls, the central macula might serve as a promising biomarker candidate reflecting SN changes. Further longitudinal studies are required to evaluate the role of these combined markers in the PD diagnostic algorithm.